CA2882008A1 - Apparatus and method for in situ assessment of thermal properties - Google Patents

Abstract

An apparatus and method for in situ assessment of the undisturbed temperature and thermal conductivity of a medium is presented. Heat is injected in the medium along short sections of electric heating cable interchanging with longer sections of electric non-heating cable. The temperature is monitored near the middle of the heating cable sections during the experiment. Observations selected over a preferred period of time are reproduced with an analytical solution describing conductive heat transfer from a linear source of finite length to identify the thermal conductivity of the medium. The use of short sections of electric heating cable allows simultaneous and multiple measurements to be performed at different locations, while maintaining a low power requirement.

Description

PATENT APPLICATION OF
JASMIN RAYMOND AND LOUIS LAMARCHE, MONTREAL, CANADA
TITLE: APPARATUS AND M ETHOD FOR IN SITU ASSESSMENT OF THERMAL
PROPERTIES
REFERENCE CITED
US Pat. Documents 3,668,297 06/1972 Howell etal.
3,864,969 02/1975 Smith, Jr.
3,892,128 07/1975 Smith, Jr.
3,981,187 09/1976 Howell 4,313,342 02/1982 Poppendiek 4,343,181 08/1982 Poppendiek 8,005,640B2 08/2011 Chiefetz et al.
Dutch Pat. Document 2009 011 600 Al 01/2006 Kietzmann et al.
European Pat. Documents 1,600,749 B1 05/2005 Rohner et al.
1,959,213 B1 02/2008 Rohner et al.
World Pat. Document 2010/058056 Al 05/2010 Martos Torres etal.
Other Documents Austin III, 1998. Development of an in situ system for measuring ground thermal properties. Oklahoma State University, Master's Thesis, Oklahoma.
Eskilson, 1987. Thermal analysis of heat extraction boreholes. Lund Institute of Technology, Doctoral Thesis, Lund.

Gehlin, 1998. Thermal response test - in-situ measurements of thermal properties in hard rock. Lulea University of Technology, Licentiate Thesis, Lulea.
Raymond, 2010. Geothermal system optimization in mining environments. Laval University, Doctoral Thesis, Quebec.
Rohner et al., 2005. A new, small, wireless instrument to determine ground thermal conductivity in-situ for borehole heat exchanger design. Proceedings of the World Geothermal Congress, Antalya, pp. 1-4.
BACKGROUND OF THE INVENTION
[0001] The present invention relates to an apparatus and method for assessing the thermal properties of a medium. More specifically, the invention relates to an apparatus used to measure temperature and inject heat in the medium and the method used to determine the undisturbed temperature and the thermal conductivity of the subsurface from the heat injection experiment performed in the field.

[0002] Several apparatus and method's have been proposed for in situ measurements of thermal properties. For example, US Pat. No. 3,668,927 discloses an apparatus in which a probe with a single heating section is lowered into a borehole to perform measurements of the surrounding thermal conductivity.

[0003] Modifications of the apparatus described in US Pat. No 3,668,927 and further methods to carry out thermal conductivity measurements in a borehole were disclosed in US Pat. Nos. 3,864,969, 3,892,128, 3,981,187, 4,313,342 and 4,343,181 as well as in Dutch Pat. No. 10 2009 011 600 Al. All these patents concern probes which have to be placed at a specific position in a hole surrounding the medium. A change of probe location along the borehole is therefore required to determine a thermal conductivity profile.

[0004] Other methods for measuring the thermal conductivity of a medium involve the use of a heat exchanger installed in a borehole, where heated liquid is circulated to disturb the thermal equilibrium of the medium. For example, US Pat. No.
8,005,64062 discloses an apparatus for performing thermal conductivity measurements, which comprises a heat exchanger connected to a surface apparatus enclosing a pump, a heating element, temperature sensors and a computer for recording data. The method used to perform the test is itself based on the thermal response test method described Page 2 of 14 in the scientific literature by Gehlin (1998) and Austin III (1998). A high potential difference electric current is required to run the apparatus. Analysis of data obtained from the test provides a global measurement of the thermal conductivity of the medium over the length that is intercepted by the borehole. Mathematical development of the analytical solutions used to analyze the tests has been described by Eskilson (1987).

[0005] Improvements of the thermal response test method have been further described in various patents. European Pat. No. 1,959,213 B1 discloses a method for measuring the thermal conductivity of a medium surrounding a borehole in which a heat exchanger is installed, and where temperature profiles are recorded at different times following heat injection by means of circulation of a heated liquid in the heat exchanger.
The evaluation of the thermal conductivity of the medium is carried out with a finite element numerical model.

[0006] World Pat. No. 2010/058056A1 discloses a method and apparatus for conducting temperature measurements with a wireless probe flowing in a heat exchanger installed in a borehole in which heated liquid is circulated. The method provides additional data for assessing the thermal conductivity of the medium at different depths with the thermal response test.

[0007] Thermal response tests to measure the thermal conductivity of a medium at different depths have also been performed with continuous heating cables lowered inside the pipes of a heat exchanger installed in a borehole. The continuous heating cables are powered with electric current and dissipate heat disturbing the thermal equilibrium of the medium. Long and continuous heating cables require a high potential difference electric current for the heat injection rate to be sufficient. The temperature is measured at different locations along the heating cables during the heat injection and during the following recovery period. The thermal response test method with continuous heating cables has been described in the scientific literature by Raymond (2010).

[0008] An accurate temperature profile measured in a borehole revealing variations of the geothermal gradient can alternatively be used to determine the thermal conductivity of the medium surrounding the borehole if the Earth's natural heat flow is known at the studied site, as described by Rohner et al. (2005). European Pat. No.
1,600,749 B1 discloses a method for measuring the temperature profile in a U-shaped heat exchanger with a wireless probe that sinks into the pipe of the heat exchanger to determine the Page 3 of 14 geothermal gradient and infer the thermal conductivity. The Earth's natural heat flow has unfortunately not been measured with significant accuracy in all regions of the world, and the method is therefore restricted to specific areas.

[0009] All these patents and scientific publications had a significant impact on the development of apparatus and methods for in situ measurements of thermal properties.
In view of the above, it will be apparent to those skilled in the art that there exists a need to improve such in situ measurements with: (1) a method that does not requires that an apparatus be moved from different positions to obtain a profile of the medium's properties, (2) an apparatus that requires low power source to decrease costs, and (3) the possibility of performing the measurements in any region of the world.
SUMMARY OF THE INVENTION

[0010] The objective of the present invention is to provide a simple apparatus and method for carrying out in situ measurements of the thermal properties of a medium using a low power source.

[0011] The apparatus of the present invention encloses short sections of heating cable along which heat is dissipated to disturb the thermal equilibrium of the medium where a hole has been drilled to insert the cable assembly. The use of short heating cable sections interchanging with longer length non-heating electric cable sections is the novel feature that makes the apparatus unique. Such an arrangement of heating cables has not been considered previously for thermal conductivity. The field and analysis methodology has been adapted to this unique feature of the apparatus, which has resulted in further innovations.

[0012] The heating cable sections allow measurements of the medium properties at different locations along the hole with decreased power requirements for testing, as compared to a conventional thermal response test with flowing liquid or a thermal response test with continuous heating cables. Measurements of thermal properties do not rely on knowledge of the Earth's heat flow, and so the new method can therefore be carried out just about anywhere.

[0013] An assessment of thermal properties is performed by reproducing temperatures recorded during the thermal recovery period following heat injection. An analytical solution describing heat transfer from a finite length linear source is used to Page 4 of 14 reproduce the temperature observations. Mathematical solutions of the prior art are linear and cylindrical heat sources, of either infinite length or finite length at one extremity and an image heat source at the other extremity. The linear heat source solution with a finite length at both extremities and its use in the scope of the present invention constitute a unique feature of the methodology.

[0014] These and other objects, novel features, aspects, advantages and further scope of applicability of the present invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the accompanying drawings, disclose a preferred embodiment and method of the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Referring now to the attached drawings which form a part of this original disclosure:

[0016] FIG. 1 is a linear view of a first type of cable assembly comprising the heating and non-heating electric cable sections for performing the test in accordance with the present invention.

[0017] FIG. 2 is a linear view of a second type of cable assembly comprising the heating and non-heating electric cable sections for performing the test.

[0018] FIG. 3 is a drawing of the electric circuit formed by the cable assembly when resistances of the heating sections are connected in series.

[0019] FIG. 4 is a drawing of the electric circuit formed by the cable assembly when resistances of the heating sections are connected in parallel.

[0020] FIG. 5 is a vertical view of the apparatus installed in a ground heat exchanger consisting of a U-pipe inside a hole.

[0021] FIG. 6 is a vertical view of the apparatus installed in a ground heat exchanger consisting of coaxial pipes inside a hole.

[0022] FIG. 7 is a vertical view of the apparatus installed in a hole sealed with a pipe or a flexible liner.

[0023] FIG. 8 is a vertical view of the apparatus installed in an open hole.
Page 5 of 14

[0024] FIG. 9 is a plan view of the objects enclosed in the apparatus at the surface of the medium.

[0025] FIG. 10 is a flowchart of the field test method for the measurement of thermal conductivity in a hole.

[0026] FIG. 11 is a flowchart of the analysis method for the measurement of thermal conductivity in a hole.
DETAILED DESCRIPTION

[0027] Referring now to the invention in more detail, in FIGS. 1 and 2, an elongated cable assembly is shown, containing sections of heating electric cable 1 interchanging with sections of non-heating electric cable 2. The sections of the heating and non-heating cables are placed one after the other, and the number and length of the sections are adjusted according to the length of the hole in which the cable assembly is installed and the number and depth of the measurements that have to be completed.
The length of all heating cable sections is equal to another and additionally shorter than the length of the non-heating cable sections. The ratio of the length of the sections of non-heating cable over the length of heating cable sections is at least higher than 2, and can vary greatly with the borehole length. For example, this ratio is preferably around 6 to 12 when the length of the borehole is approximately 150 m. The outer jacket at the ends of the sections of heating and non-heating cables can be attached to a means of connection 3, as shown in FIG. 1 or can be continuous, avoiding the means of connection, as shown in FIG. 2. Both cable assembly types are terminated with a waterproof male electric connector at one extremity 4 and with a submersible end-seal 5 that closes the electric circuit at the other extremity. At least one perforated disk 6 is located at the interface of each cable section. The size of the perforations must be large enough to allow water to flow through the disks when installing the cable assembly into a hole, and must be small enough to block water movements due to free convection when tests are conducted. For example, the size of the perforations should be more than 1 mm in diameter and less than 1 cm in diameter. Each cable interface can enclose more than one disk 6, depending on the size of the perforations and the capacity of the disks to block water movements due to free convection.
Perforations should preferably be offset if more than one disk is installed at each cable interface.
Page 6 of 14

[0028] At least one means of measuring the temperature 7 is located near the middle height of each heating cable section. The location of the means of measuring the temperature near the middle height of the heating cable sections is such as to facilitate the reproduction of measured temperatures with an analytical solution of finite length at both extremities. There can be more than one means of measuring the temperature per heating cable section, all located at the middle height or at different locations along the heating cable section to duplicate or to allow more detailed measurements. The means of measuring the temperature can involve: (1) submersible capsules, enclosing a temperature sensor and a data logger, (2) thermistors connected to a data logger at the surface with wires, or (3) a fiber optic cable connected to an optical reader and data logger at the surface. The preferred means of measuring the temperature is the submersible capsule, which avoids using the several wires required with thermistors.
Furthermore, the submersible capsule can easily be placed near the middle height of the heating cable sections. Carrying out temperature measurements near the middle height of the heating cable sections can be complex with a fiber optic cable whose spatial resolution is commonly around 1 m.

[0029] Referring in more detail to FIGS. 3 and 4, the electric circuit of the cable assembly is shown to enclose a power source 8, electric wires or conductors 9 and electric resistances 10, which constitute the heating cable sections. The electric resistances 10 are made with thin electric wires that can be connected in series, as shown in FIG. 3 or in parallel, as shown in FIG. 4. The electric resistances of the heating cable sections R1, R2, Rn are equal to each other, and the difference in potential at the power source is V. The power outputs Q1, Q2, ..., Qn for the circuit with series resistances shown in FIG 3 are equal to each other since the current intensity / is the same throughout the circuit. The power outputs Q1, Q2, Qn for the circuit with parallel resistances shown in FIG 4 are different from one another since the current intensities /1, /2, .../n are different for each section. Series connections are preferred over parallel connections for the resistances to have the same power output for each heating section.

[0030] When the electric resistances of the heating cable sections are connected in series, as shown in FIG. 3 or in parallel, as shown in FIG. 4, the means of connection 3 between the heating and non-heating cable sections can be made with submersible Page 7 of 14 splices or connectors, as shown in FIG. 1. The use of splices or connectors can be avoided, as shown in FIG. 2, when resistances are connected in parallel, as shown in FIG. 4. In this case, the thin wire used for the parallel resistances of the heating sections can be rolled around the two conductors through the whole cable assembly, but cut to the desired length and brought in contact with the two conductors at the locations of the heating sections only. A continuous outer jacket is extruded over the thin wire for the cable assembly to be submersible. A cable assembly without splices or connectors, as shown in FIG. 2, can also be fabricated by extrusion of an outer jacket over the heating and non-heating cable sections connected in series, as shown in FIG.
3. In this case, the inside jacket around the thin wires of the heating cable is made thicker to allow the heating and non-heating sections to have the same diameter. The preferred cable assembly is that of FIG. 1, and submersible connectors constitute the preferred means of connection 3 because the number of heating and non-heating sections can be easily adjusted from one test to another.

[0031] Referring in more detail to FIGS. 5, 6, and 7, which illustrate the installation of the apparatus in a hole 11 cutting across the medium, we see the medium surface 12, an optional pipe casing 13 to hold the medium in place when it is non-consolidated and material 14 filling the hole 11. Most commonly, the medium is the ground or the earth subsurface, but it could also be a structure of various material types, such as a concrete dam. The hole 11 can be installed by drilling, which makes it a borehole, or by any other means, while constructing the structure that constitutes the medium. Material 14 that fills the hole 11, which is shown to be vertical, and can also be inclined, is most commonly grout, but could also be water, sand, or backfill material of any kind. The cable assembly is connected to a waterproof electric junction box 15 with the waterproof male electric connector 4. The junction box can be hung to the pipe casing 13 with rigid pipe straps 16. An electric cable 17 with waterproof female 18 and male 19 connectors is used to connect the junction box 15 to a power source.

[0032] The cable assembly can be placed in the pipe of a hole where a ground heat exchanger made with a single U-pipe 20 or multiple U-pipes has been installed, as shown in FIG. 5. The pipe is filled with a liquid whose level is illustrated by the dash lines and triangles 21. The liquid is most commonly water, but could also be a mix of water and antifreeze or other aqueous solution of any kind. The ground heat exchanger Page 8 of 14 can alternatively be made with a center pipe of smaller diameter 22 surrounded by a pipe of larger diameter 23 in a coaxial fashion, as shown in FIG. 6. In this case, it is better to place the cable assembly in the smaller diameter center pipe 22. The coaxial pipes are filled with liquid whose level is illustrated by the dash line and triangle 21. The cable assembly can also be installed in a hole 11 that does not include a ground heat exchanger, as shown in FIG 7 and 8. The hole 11 can be blocked with a solid pipe or a flexible liner 24 filled with a liquid whose level is illustrated by the dash line and triangle 21, as shown in FIG. 7. The hole 11 can alternatively be in open contact with the medium, as shown in FIG. 8. The level of the liquid in the hole, illustrated through the dashed line and triangle 21, will be equal to that of the medium if there is liquid in the pores or fractures of the medium at an unconfined pressure. The level of the liquid in the hole could be different from that of the medium if the pressure of the liquid in the pores or fractures of the medium is confined. The heating cable sections are placed below the level of the liquid in the hole or pipe to avoid convective heat transfer in the air. The width of the perforated disks 6, which are used to block water movements due to free convection, are adjusted according to the inner diameter of the pipes 20, 22 (FIGS 5 and 6), liner 24 (FIG 7) or hole 11 (FIG 8). The width of the perforated disk should be small enough for the disks to enter the pipes, liner or hole, and large enough to block water movements due to free convection in the open space between the disk and the inner surface of the pipes, liner or hole.

[0033] Referring in more detail to FIG. 9, which illustrates the main components of the waterproof electric junction box 15, there is shown a breaker panel 25, an automated switch or a timer 26 for programming the system operation, a power meter and data logger 27 for recording measurements, a handle 28 for transporting the box, and rigid pipe straps 16 and bolts 29 to hang the box to the pipe casing of the hole.
Electric power enters the box through the electric cable 17 with a waterproof female connector 18 installed in a flanged inlet 30. An electric cable 31 links the flanged inlet 30 to the breaker panel 25 including at least two electric circuits. One of the circuits 32 supplies the power meter and data logger 27. The other circuit exits the breaker panel through an electric cable 33 that goes into the automated switch or timer 26, then through another electric cable 34 connected to the reading ports of the power meter and data logger 27, and then through an electric cable 35 connected to a flanged outlet 36.
Page 9 of 14 Electric power exits the box through the waterproof male connector 4 and the first non-heating section 2 of the cable assembly installed in the hole to perform the test.

[0034] Referring in more detail to FIG. 10, which illustrates the field method for carrying out thermal conductivity measurements of the medium, there are five basic steps to follow. The test begins by setting and starting the data loggers to record temperature measurements near the middle height of the sections of heating cable 1 as well as potential differences and the current intensity of the electric circuit formed by the cable assembly shown in FIGS. 3 and 4. The cable assembly shown in FIGS 1 or 2 is installed inside the hole according to FIGS 5, 6, 7 or 8. The electric junction box 15 is fixed to the pipe casing 13 with pipe straps 16, the cable assembly is hooked up to the electric junction box 15, which is connected to a power source with the electric cable 17.
The undisturbed temperature of the medium is measured and recorded at different depths inside the hole using the means of measuring temperature 7 before starting heat injection. The switches of the breaker panel 25 are turned on to set the automated switch or timer 26 and allow the electric current to flow in the cable assembly to begin heat injection. The difference in potential and the current intensity of the electric circuit of the cable assembly are measured and recorded with the power meter and data logger 27. The duration of heat injection in the medium is approximately 50 hours. This heat injection period can be longer for a medium of low thermal conductivity and shorter for a medium of high thermal conductivity. Heat injection is stopped and the temperature is measured and recorded at different depths inside the hole using the means of measuring temperature 7 for a duration that is roughly equivalent to that of the heat injection period. Equipment is retrieved after the thermal recovery, and all recorded data are downloaded, ending the field test procedure.

[0035] Referring in more detail to FIG. 11, which illustrates the analysis method for carrying out thermal conductivity measurements of the of the medium, there are five basic steps to follow, one of which is decisional. After the field procedure is ended, the undisturbed temperatures at the depths of the means of measuring temperature 7 are evaluated from the measurements taken before heat injection. Temperature increments, defined as the temperature difference between a given time and the undisturbed condition (AT=T-To), are determined from measurements at depth taken after the beginning of the heat injection. Observed temperature increments at depth are then Page 10 of 14 reproduced over a selected time interval. Only the temperature increments during the late recovery period are reproduced. Data recorded early following the end of the heat injection must be removed from observations to reproduce until the temperature through the middle horizontal plane of each heating cable section becomes uniform near the cable. This period of time is about a few hours, and can be pre-determined with numerical simulations of heat transfer for selected thermal properties of the medium and borehole materials. Observed temperature increments at depth during the selected time interval are reproduced with an analytical model solving conductive heat transfer from a linear heat source of finite length. Calculated temperature increments are fitted with observed temperature increments by adjusting the thermal conductivity of the medium.
A verification of the quality of the fit between observed and computed temperature increments is done. If the fit is satisfactory, the undisturbed temperature and thermal conductivity measurements at the given depths can be reported; otherwise, analysis starts over again at the third step.

[0036] The advantage of the present invention is that it offers the possibility of assessing the thermal conductivity of a medium at several locations without moving an apparatus at different depths into a hole, and conducting repetitive tests at each location. A single heat injection experiment with multiple heat sources and temperature sensors is carried out with the present invention. The power source required to perform the test can be of low potential difference because heat is injected along short heating cable sections with at least one temperature sensor per section. A test with a continuous heating cable in a long hole would require a higher power source, which complicates installation of the apparatus in the field and safety procedures.
For the present invention to be efficient, it is therefore important for the heating cable sections to be shorter than the non-heating cable sections, such as to minimize energy consumption. Short heating cable sections allow a high number of measurements that can be performed, for example, every 10 m. The spacing of heating cable sections is however not restricted to such a specific value, and can be adjusted to site settings. The cable assembly of the present invention is consequently designed to facilitate operation of the apparatus and to maximize the number of measurements, distinguishing the cable assembly from those of other fields using heat tracing technologies.
Page 11 of 14

[0037] Data collected during the field experiment could not have been analyzed with mathematical solutions and methods of the prior art since temperature buildup along a linear heat source of short finite length is significantly different from that of heat sources of infinite length or of finite length with an overlying image source. The use of a new mathematical solution with a linear heat source of finite length at both extremities in the scope of the present invention makes the test analysis comprehensive, fast and accurate. The solution is derived to describe temperature increments at the middle height of the heat source, and analysis is facilitated by positioning the temperature sensors near this location. =

[0038] The Earth's natural heat flow does not affect analysis when the geothermal gradient along the hole axis is small enough to be considered negligible.
Therefore, the test can be carried out in about any regions of the world, even where knowledge of the Earth's heat flow is poor.

[0039] While the foregoing written description of the invention enables one of ordinary skill to make and use what is considered presently to be the best mode thereof, those of ordinary skill will understand and appreciate the existence of variations, combinations, and equivalents of the specific embodiments, methods, and examples herein. The invention should therefore not be limited by the above described embodiments, methods, and examples, but by all embodiments and methods within the scope and spirit of the invention. It is therefore intended that the appended claims encompass any such embodiments, methods and examples.
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Claims (13)

The invention claimed is:

1. An apparatus used to measure the thermal properties of a medium characterized by heat injection through a cable assembly with several short electric heating cable sections.

2. An apparatus used to measure the thermal properties of the said medium characterized in that the said electric heating cable sections of the said cable assembly are interchanging with longer electric non-heating cable sections, with the ratio of the length of non-heating cable over the length of heating cable being at least 2.

3. An apparatus used to measure the thermal properties of the said medium characterized in that the said electric heating cable sections of the said cable assembly are of the same length.

4. An apparatus used to measure the thermal properties of the said medium characterized in that the interface of the said sections of electric heating and non-heating cables of the said cable assembly are separated by at least one perforated disk.

5. An apparatus used to measure the thermal properties of the said medium characterized in that at least one means of measuring temperature is placed near the middle height of each of the said electric heating cable sections of the said cable assembly.

6. An apparatus according to claims 1, 2 or 3 enclosing at the surface of the said medium, a power meter, a data logger and a switch inside a junction box.

7. An apparatus according to claims 1, 2 or 3 characterized in that the said switch is automated.

8. An apparatus according to claims 1, 2 or 3 characterized in that the said junction box has a means for attaching the said junction box to a pipe casing.

9. A method to measure the thermal properties of the said medium with the said apparatus characterized in that the said cable assembly with the said means of measuring temperature are lowered into a hole installed in the said medium to measure the temperature at different depths to determine initial conditions.

10. A method for measuring the thermal properties of the said medium with the said apparatus having the said cable assembly and the said means of measuring temperature installed in the said hole characterized in that heat is injected at a measured rate during a measured amount of time.

11. A method for measuring the thermal properties of the said medium with the said apparatus characterized in that the said temperature is measured during the recovery period following the injection of heat.

12. A method for measuring the thermal properties of the said medium with the said apparatus characterized in that the said temperature measured during the said period of thermal recovery is reproduced over a selected time interval with an analytical solution solving conductive heat transfer from a linear heat source of finite length at both extremities by adjusting the thermal properties of the said medium.

13. A method for measuring the thermal properties of the said medium with the said apparatus characterized in that the said analytical solution solving for conductive heat transfer is derived to describe temperature increments at the middle height of the said heat source of finite length at both ends.